Smallpox was eradicated through a worldwide effort coordinated by the World Health Organization (WHO) global vaccination campaign in the second half of the last century (Fenner F. et Al., 1988). This disease was “the most dreadful scourge of the human species” (Fenner F., 1984) and claimed hundreds of millions of victims for centuries (Fenner F. et AL., 1988). Variola virus (VARV), its causative agent, spreads easily and exclusively from human to human by the respiratory route. It causes fever, severe rash and in about 30% of cases, death (Fenner F., et Al., 1984).
The fear of the release of VARV through bioterrorism has generated renewed interest in the prevention of smallpox because of the high proportion of unimmunised people in the global population and because vaccination is the only currently effective means to curtail a smallpox epidemic. Live vaccinia virus (VACV) is the active ingredient of the smallpox vaccine administered by scarification. VACV and VARV belong to the Orthopoxvirus genus within the family Poxyiridae and both these viruses display considerable serological cross-reactivity, allowing VACV to provide protection from VARV infection, the accepted basis of its use as a smallpox vaccine. In view of the smallpox threat, a number of countries have maintained stockpiles of the first-generation smallpox vaccine. Inevitably, there will be a need to replace or to increase the stockpiles, but the historical manufacturing process, in the skin of live animals is no longer acceptable. This has stimulated interest in developing second-generation vaccines made of live, replicative, vaccinia virus, but manufactured by virus replication in cell cultures.
Several new second-generation smallpox vaccines have been developed using tissue culture-adapted virus: one such vaccine (ACAM2000™, a live vaccinia virus smallpox vaccine) is derived from a New York City Board of Health (NYCBH) strain first-generation vaccine through cloning and propagation in MRC-5 and Vero cell cultures (Monath T P, et Al., 2004) (Weltzin R, et Al., 2003), and others are derived from a Lister/Elstree first-generation vaccine without cloning.
Second-generation vaccines have the advantage over first-generation vaccines of being produced and controlled according to Good Manufacturing Practices (GMP), thus being more standardised and free of adventitious agents. Nevertheless, these second-generation vaccines are unsatisfactory because they may still induce the same vaccine complications as those induced by the first generation vaccines.
H. Mahnel and colleagues passaged the vaccinia virus Ankara strain (CVA) more than 500 times in chicken embryo cells and isolated a highly attenuated vaccine named MVA (Mahnel, H. and Mayr, A., 1994). During the multiple passages of the CVA virus in tissue culture that ultimately led to the MVA virus, 6 main regions of the viral genome were deleted and numerous point mutations and smaller deletions occurred (Antoine, G., F. et Al., 1998 and Meyer, H., et Al., 1991; Meisinger-Henschel et AL., 2007)
The MVA strain has been extensively characterised and has been found to be efficacious in protecting animals from challenge infections mimicking smallpox and to display a very promising profile as a smallpox vaccine in clinical trials. Nevertheless, potential drawbacks of the MVA smallpox vaccine lie in the fact that it must be employed at very high doses (108 PFU/injection intramuscularly or intradermally, a dose more than 100 fold higher than the dose used with the first generation smallpox vaccine) because it does not replicate in human cells and a booster vaccination is recommended to achieve long-lasting immunity. Furthermore, the MVA vaccine produces no visual take at the site of inoculation as produced by the traditional smallpox vaccine.
Therefore, there is still an important need for new viral strains derived from the vaccinia virus which have better vaccine potency at a lower dose than the MVA strain dose.